1. Field of the Invention
The present invention relates to a coated substrate with high reflectance. It is especially concerned with transparent glass substrates bearing a coating of oxides of tin and antimony and with the use of such substrates in exterior glazing panels for buildings.
2. Description of the Related Arts
Although architects seeking glazing panels for use in buildings have traditionally tended to favour panels with low levels of reflection, a changing perception of the aesthetic appeal has led to increasing demands for panels with higher levels of reflection but without the glare as viewed from outside which is associated with very high levels of reflection. The panels may also be required to have other qualities such as providing protection for occupants of the building against solar radiation and the associated overheating (solar screening properties).
The panels comprise at least one sheet of a transparent substrate material, typically soda-lime glass, with a thin coating on one or more of the sheet faces to modify the optical and physical properties of the sheet and the panel as a whole. A huge variety of prior proposals have been made for the coating, according to the specific properties sought. The coating may comprise a stack of several discrete layers chosen with appropriate compositions and thicknesses to complement their respective effects. A persistent problem in choosing the respective layers is that a layer adopted for one purpose may adversely change the effect of other layers.
Tin oxide (SnO2) has been widely used as a coating material, often in combination with other metal oxides. Coatings comprising tin oxide with a small proportion of antimony oxide have proved especially attractive.
Our GB patent 1455148 teaches a method for pyrolytically forming a coating of one or more oxides (e.g. ZrO2, SnO2, Sb2O3, TiO2, Co3O4, Cr2O3, SiO2) on a substrate, primarily by spraying compounds of a metal or silicon, so as to modify the light transmission and/or fight reflection of the substrate Our GB patent 2078213, which relates to a method for pyrolytically forming a coating by two separate sprays to achieve high rates of coating build-up, discloses tin oxide coatings doped with fluorine or antimony. Our GB patent 2200139 relates to forming a pyrolytic tin oxide coating from a precursor containing at least two additives such as oxidising agents, sources of fluorine and sources of metal.
The use of a tin oxide coating with a small proportion of antimony oxide has been found to offer several advantageous combinations of optical and energy properties Our GB patent applications 2302101 (""101) and 2302102 (""102) describe anti-solar glazing panels comprising a pyrolytic coating layer of oxides of tin and antimony in which the Sb/Sn molar ratio is from 001 to 0.5. The ""101 coating is applied by liquid spray and has a thickness of at least 400 nm, a luminous transmittance of less than 35% and a selectivity of at least 1.3. The ""102 coating is applied by chemical vapour deposition (CVD) and has a solar factor below 70%.
The use of pyrolysis to form a coating on a substrate generally has the advantage of producing a hard coating with durable abrasion-resistant and corrosion-resistant properties. It is believed that this is due in particular to is the fact the process involves deposition of coating material on to a substrate which is hot Pyrolysis is also generally cheaper than alternative coating processes such as sputtering, particularly in terms of the investment in plant.
Properties of the coated substrate discussed herein are based on the standard definitions of the International Commission on Illuminationxe2x80x94Commission Internationale de l""Eclairage (xe2x80x9cCIExe2x80x9d). The illuminant for the rests was illuminant C, which represents average daylight having a colour temperature of 6700 K and is especially useful for evaluating the optical properties of glass intended for use in buildings.
The xe2x80x9cluminous transmittancexe2x80x9d (TL) is the luminous flux transmitted through a substrate as a percentage of the incident luminous flux.
The xe2x80x9cluminous reflectancexe2x80x9d (RL) is the luminous flux reflected from a substrate as a percentage of the incident luminous flux.
The xe2x80x9cpurityxe2x80x9d (p) of the colour of the substrate refers to the excitation purity in transmission or reflection.
The xe2x80x9cdominant wavelengthxe2x80x9d (xcexD) is the peak wavelength in the transmitted or reflected range.
The xe2x80x9csolar factorxe2x80x9d (FS), referring to the transmission of total incident solar radiation through the coated substrate, is the sum of the total energy directly transmitted (TE) and the energy which is absorbed and re-radiated on the side of the coated substrate away from the energy source, as a proportion of the total incident radiant energy.
The xe2x80x9cselectivityxe2x80x9d of a coated substrate for use in a building glazing panel is the ratio of the luminous transmittance to the solar factor it is an object of the present invention to provide a pyrolytically formed coating on a substrate to impart solar screening properties and a high reflectance to the substrate.
We have discovered that this and other useful objectives can be achieved by depositing a coating stack comprising a defined overcoat layer on a main layer comprising tin and antimony oxides.
According to the invention there is provided a transparent substrate carrying a coating stack comprising a pyrolytically-formed main layer containing oxides of tin and antimony, characterised in that the main layer has a geometric thickness of at least 250 nm and in that the stack includes an outer reflective layer having a geometric thickness in the range 30 to 150 nm and having a refractive index in the range 2.0 to 2.8. whereby the so-coated substrate has a reflectance (RL) of more than 10%.
The presence of the outer reflective creates an improvement in the luminous reflectance (RL) of the coated substrate, increasing the reflectance from less than 10% to more than 10%, and generally to at leas 15% and even to around 25%. Moreover these increases are achieved without taking the other optical properties of the substrate beyond acceptable limits. The outer layer is also beneficial in further improving the abrasion and corrosion resistance of the coating.
Although the invention is described herein primarily with reference to glazing panels for buildings, panels according to the invention are suitable for other applications such as vehicle windows, in particular vehicle sunroofs.
Preferably the outer reflective layer contains an oxide of one or more of nickel, tin, titanium, zinc and zirconium. These materials readily form by pyrolysis a coating with the required refractive index.
The outer reflective layer preferably comprises oxide of titanium. This gives a high luminous reflectance for a very thin coating thickness. Preferably, the coating contains oxide of titanium together with oxide of tin. This confers to the coating a better abrasion and chemical resistance. Such a coating contains most preferably at least 50% by volume of tin oxide and at least 30% by volume of titanium oxide. The preferred geometric thickness for a titanium oxide coating is in the range 45-55 nm. The preferred geometric thickness for tin/titanium oxide reflective layer is in the range 40 to 75 nm. Below 40 nm the layer may not be sufficient to modify the optical properties, especially the reflectance, of the coated product. Above 75 nm the level of luminous reflection may be unduly high and the optical effects of the overcoat will tend to mask the optical effects of the other layers in the stack. More preferably the said layer has a thickness in the range 60 to 75 nm. This range permits the attainment of good optical stability for the coating stack. Optical stability means that variations of the thickness of the layer, inherent in industrial production, do not cause significant changes of the optical properties, particularly of Hunter values a and b and purity in reflection. Optical stability is even better when the overcoat has a thickness between 60 and 70 nm.
The Sb/Sn oxide materials of the main layer impart good anti-solar properties to the coated substrate The geometric thickness of at least 250 nm for this layer represents the optimum range for a layer in terms of providing solar screening properties sought after and a neutral tint. Preferably the said thickness is lower than 650 nm, for economic and practical reasons. Most preferably, the thickness is in the range 300 to 360 nm. Such a range permits the attainment of coated products with sufficient solar screening properties and presenting optical stability.
Preferably the coated product has a Hunter value a between 0 to xe2x88x922, and Hunter value b between xe2x88x924 and xe2x88x922, thus meaning a slightly bluish aspect in reflection. Purity in reflection is preferably low, i.e. less than 10%, preferably between 4 and 7.5.
As taught in our earlier patent specification GB-A-2302102 the Sb/Sn molar ratio in the main coating layer is preferably in the range 0.01 to 0.5, more preferably in the range 0.03 to 021.
As described and claimed in our copending patent application of the same date as the present application, the reflectance of the coated substrate may be further improved by also including in the main coating layer an additive comprising one or more of aluminum, chromium, cobalt, iron, manganese, magnesium, nickel, titanium, vanadium, zinc and zirconium. The said additive is preferably selected from chromium iron and magnesium.
In one embodiment of the invention the coating stack further comprises an undercoat positioned between the substrate and the main coating layer, The undercoat serves to improve the aesthetic appeal of the coating both by reducing or eliminating haze in the coating stack and by neutralizing the colour that the tin oxide in the main layer tends to impart to the stack.
Suitable materials for the undercoat include one or more silicon oxide or alumina-based coating, for example alumina with a small proportion of vanadium oxide. In the case of silicon oxides it is preferred to use an incompletely oxidised material i.e. SiOx where x is less than 2, which may have the general structure of SiO2 but has a proportion of gaps which would be filled with oxygen in the dioxide. This can be achieved by employing oxygen in an insufficient quantity for full oxidation of the undercoat material on the substrate,
The preferred geometric thickness of the undercoat is in the range 60 to 75 nm. This is the range in which the undercoat tends best to impart to the coating stack a neutral tint in reflection.
In a further embodiment of the invention the coating stack also comprises an intermediate layer positioned between the main coating layer and the outer reflective layer. This intermediate layer is a further means of increasing the luminous reflectance of the coated substrate. Suitable materials for the intermediate layer include oxides of aluminum or silicon, which may be used alone or in admixture.
Because the presence of fluorine tends to hinder the incorporation of certain elements such as antimony into the coating layers it is desirable that fluorine be excluded from the coating layers of the invention.
Preferably, as discussed above, the reflectance (RL) of the coated substrate is at least 15% but not so great as to create glare in reflection. Thus it is preferred that the coated substrate has a maximum reflectance (RL) of 25%, most preferably a maximum reflectance of 20%.
It is mostly required that the glazing panel shall transmit a sufficient proportion of visible light in order to allow both good natural illumination inwards into the building or vehicle and good visibility outwards. The light transmittance (TL) of a coated substrate according to the invention is preferably greater than 60%.
It is desirable to increase to a high level the selectivity of the coating, i.e. the ratio of the transmittance to the solar factor. It is preferred that the selectivity is greater than 1,00.
The invention includes within its scope a glazing panel comprising a coated transparent substrate as defined herein, The panel may be a single sheet or alternatively may include two or more substrate sheets in a multiple-glazed or laminated assembly. In a multiple glazing or laminated assembly it is preferred that just one of the constituent sheets carries the coating.
Pyrolytic methods are generally preferred for the application of all the layers of the coating stack of the invention, Coatings produced by pyrolysis are generally advantageous in having a greater mechanical resistance than coatings produced by other methods. The reactant materials to be pyrolysed may be applied to the substrate by chemical vapour deposition (CVD or xe2x80x9cvapour pyrolysisxe2x80x9d) or as a liquid spray (xe2x80x9cliquid pyrolysisxe2x80x9d)
Application of a pyrolytic coating to flat glass is best achieved when the glass is newly formed, e.g. as it leaves a float glass line. This provides economic benefits in avoiding the need to reheat the glass for the pyrolytic reactions to take place, and in the quality of the coating, since the newly formed glass surface is in pristine condition.
Preferably the source of tin for the main layer is selected from SnCl2, SnCl4, Sn(CH3)2Cl2, tetramethyl tin or monobutyl trichloro tin (xe2x80x9cMBTCxe2x80x9d). The source of antimony for the main layer may be selected from SbCl5, SbCl3, organo antimony compounds such as Sb(OCH2CH3)3, Cl1.7Sb(OCH2CH3)1.3, Cl2SbOCHClCH3, Cl2SbOCH2CHCH3Cl and Cl2SbOCH2C(CH3)2Cl. The source of any metallic additive for the main layer may similarly be a suitable chloride or organo-metallic compound of the respective metal.
The sources of reactants for be respective layers are preferably formed into single staring matures for each of the layers, whereby all of the starting reactants for a given layer are applied simultaneously to the substrate,
To form a coating layer by CVD, the respective reactant mixture is applied, typically through a nozzle, to the substrate in a coating chamber. Where this mixture comprises chlorides which are liquid at ambient temperature, it is vaporised in a heated current of anhydrous carrier gas such as nitrogen. Vaporisation is facilitated by the atomization of these reagents in the carrier gas. To produce the oxides, the chlorides are brought into the presence of a source of oxygen, for instance, water vapour.
Methods and devices for forming such a coating are described for example in French patent No 2348166 or in French patent application No 2648453. These methods and devices lead to the formation of particularly strong coatings with advantageous optical properties.
To form the coating by a spray method, the substrate may be brought into contact with a spray of droplets containing the respective reactant materials. The spray is applied by one or more spray nozzles arranged to follow a path which provides the coating across the width of the ribbon to be coated.
CVD offers benefits over sprayed liquids in providing coatings of regular thickness and composition, such uniformity of the coating being important where the product is to cover a large area. A spray coating also tends to retain traces of the sprayed droplets and of the path of the spray gun. Moreover, the pyrolysis of sprayed liquids is essentially limited to the manufacture of oxide coatings, such as SnO2 and TiO2. It is also difficult to make multi-layer coatings using sprayed liquids because every coating deposition produces a significant cooling of the substrate, Furthermore, CVD is more economic in terms of raw materials, leading to lower wastage.
However despite such disadvantages of the spray method it is nevertheless convenient and inexpensive to apply and employs simple equipment It is thus often adopted, especially for formation of thick coating layers.
Glazing panels incorporating coated substrates according to the invention may be manufactured as follows. Each pyrolytic coating step may be carried out at a temperature of at least 400xc2x0 C., ideally from 550xc2x0 C. to 750xc2x0 C. The coatings can be formed on a sheet of glass which moves in a tunnel oven or on a glass ribbon during formation, whilst it is still hot. The coatings can be formed inside the lehr which follows the glass ribbon forming device or inside the float tank on the top face of the glass ribbon whilst the latter is floating on a bath of molten tin.